Martian Meteorites

NASA scientists now believe that more than 30 unusual meteorites are actually pieces of Mars that were blasted off the Red Planet by meteor-oid impact collisions. These Martian meteorites were previously called

SNC meteorites, after the first three types of samples discovered (namely: Shergotty, Nakhla, and Chassigny). The Chassigny meteorite was discovered in Chassigny, France, on October 3, 1815. It establishes the name of the chassignite-type subgroup of the SNC meteorites. Similarly, the Shergotty meteorite fell on Shergotty, India, on August 25, 1865, and provides the name of the shergottite-type subgroup of SNC meteorites. Finally, the Nakhla meteorite was found in Nakhla, Egypt, on June 28, 1911, and establishes the name for the nakhlite-type subgroup of SNC meteorites. Another member of the Martian meteorite family, the 40-pound (18-kg) Zagami meteorite fell to Earth on October 3, 1962, near Katsina, Nigeria.

Martian meteorites have been found on every continent, except Australia. Unlike some meteorites that landed elsewhere and became unprotected commercial items for collectors, the samples found in Antarctica are collected and controlled by government organizations and curated for research by professional scientists.

All of the Martian meteorites are igneous rocks that have crystallized from molten lava in the crust of the parent planetary body. The meteorites that have been linked to Mars and that have been discovered on Earth so far represent five different types of igneous rocks, ranging from simple plagioclase-pyroxene basalts to almost monomineralic cumulates of pyroxene or olivine.

The only natural process capable of launching Martian rocks to Earth is meteoroid impact. To be ejected from Mars, a rock must reach a velocity of at least 3.1 miles per second (5 km/s)—the escape velocity for Mars. During a large meteoroid impact on the surface of Mars, the kinetic energy of the incoming cosmic impactor causes shock deformation, heating, melting, and vaporizing, as well as crater excavation and ejection of target material. The impact and the shock environment of such a collision provide scientists with an explanation as to why the Martian meteorites are all igneous rocks. Martian sedimentary rocks and soil would not be consolidated sufficiently to survive the impact as intact rocks and then wander through space for millions of years and eventually land on Earth as meteorites.

One particular Martian meteorite, called ALH84001, has stimulated a great deal of interest in the possibility of life on Mars. In August 1996, a NASA research team at the Johnson Space Center (JSC) announced that they had found four lines of evidence in ALH84001 that "strongly suggests primitive life may have existed on Mars more than 3.6 billion years ago." The NASA research team found the first organic molecules thought to be of Martian origin; several mineral features characteristic of biological activity; and possibly microscopic fossils of primitive, bacterialike organisms inside an ancient Martian rock that fell to Earth as a meteorite. While the NASA research team did not claim that they had conclusively proved

This 4.5-billion-year-old rock, labeled meteorite ALH84001, is believed at one time to have been a part of Mars. Some scientists speculate that this "Martian meteorite" may also contain fossil evidence that primitive life once existed on the Red Planet— perhaps more than 3.6 billion years ago. The rock is a portion of a meteorite that was apparently dislodged from Mars by a huge impact about 16 million years ago, slowly wandered through space, and then fell to Earth in Antarctica some 13,000 years ago. The meteorite was found in Antarctica's Allan Hills ice field in 1984. Today, the interesting rock is being preserved for scientific study at the Johnson Space Center's Meteorite Processing Laboratory. (NASA/JSC)

This 4.5-billion-year-old rock, labeled meteorite ALH84001, is believed at one time to have been a part of Mars. Some scientists speculate that this "Martian meteorite" may also contain fossil evidence that primitive life once existed on the Red Planet— perhaps more than 3.6 billion years ago. The rock is a portion of a meteorite that was apparently dislodged from Mars by a huge impact about 16 million years ago, slowly wandered through space, and then fell to Earth in Antarctica some 13,000 years ago. The meteorite was found in Antarctica's Allan Hills ice field in 1984. Today, the interesting rock is being preserved for scientific study at the Johnson Space Center's Meteorite Processing Laboratory. (NASA/JSC)

that life existed on Mars some 3.6 billion years ago, they did believe that "they have found quite reasonable evidence of past life on Mars."

Martian meteorite ALH84001 is a 4.2-pound (1.9-kg), potato-sized igneous rock that has been age-dated to about 4.5 billion years—the period scientists believe when the planet Mars formed. This rock is thought to have originated underneath the Martian surface and to have been fractured extensively by impacts as meteorites bombarded the planet during the early history of the solar system. Between 3.6 and 4.0 billion years ago, Mars is thought to have been a warmer and wetter world. Martian water could have penetrated fractures in the subsurface rock, possibly forming an underground water system. Since the water was saturated with carbon dioxide from the Martian atmosphere, carbonate materials were deposited in the fractures.

This high-resolution scanning electron microscope image shows an unusual tubelike structure (less than 1/100th the width of a human hair in size) found in meteorite ALH84001—a meteorite believed to be of Martian origin. Although this structure is not part of the research reported by NASA exobiologists in summer 1996, the object is located in a similar carbonate glob in the meteorite. This structure remains the subject of scientific investigations and of debates concerning whether or not it is fossil evidence of primitive life on Mars some 3.6 billion years ago. (NASA/JSC)

The NASA research team estimated that this rock from Mars entered Earth's atmosphere about 13,000 years ago and fell in Antarctica as a meteorite. ALH84001 was discovered in 1984 in the Allan Hills ice field of Antarctica by an annual expedition of the National Science Foundation's Antarctic Meteorite Program. It was preserved for study at the NASA JSC Meteorite Processing Laboratory, but its possible Martian origin was not fully recognized until 1993. It is the oldest of the Martian meteorites yet discovered.

Other scientists have used data from the Mars Global Surveyor and Mars Odyssey 2001 spacecraft in an attempt to find a reasonable candidate location on Mars from which this very controversial ALH84001 meteorite may have originated. In 2005, they reported that ALH84001 may have come from the Eos Chasma, a branch of the very long and large Valles Marineris canyon system. The most likely candidate area is a 12.4-mile (20-km) diameter crater in a lobate flow region—the type of geologic phenomenon that occurs when a high-velocity impactor strikes a fluid-rich soil. The site has 2.5-mile (4-km)-high cliffs, bordering the canyon and exposing rocks from various geologic ages in the history of Mars. The scientists further suggest that ALH84001 may have first formed in this region deep beneath the Martian surface and then was later transported to the shallow depth from which an ancient, high-speed impactor launched the famous rock into space.

However, the birthplace on Mars of ALH84001 is still a matter of much conjecture, as is the hypothesis that it contains fossilized evidence suggesting there once was primitive life on Mars. In any event, Eos Chasma appears to be an interesting region from which to sample rocks from a wide range of ages in the history of Mars and could become a prime candidate target for a future robotic or human landing mission.

More than a decade after NASA scientists at the Johnson Space Center made their startling announcement about ALH84001, the question of life on ancient Mars remains a wide-open question. The only aspect about this controversial rock about which almost all scientists agree is that it came from Mars and that it is older than any known rock on Earth. ALH84001 was volcanically formed; was determined to be incredibly old—almost as old as the solar system itself; has the appropriate elemental fingerprints for Mars—consistent with the chemistry data recorded by the Viking 1 and 2 landers in 1976; contains materials that are rare on Earth; and, finally, appears to have been exposed to flowing water at some point before it was hurled off the Red Planet. Whether this interesting Martian meteorite actually contains evidence that is conclusively suggestive of ancient biogenic activity on Mars remains an open, bitterly contested scientific debate.

Some scientists currently hold the position that neither ALH84001 nor any other Martian meteorite will lead them to establish an irrefutable, scientific conclusion about whether life actually existed on Mars. What these exobiologists want, and may someday obtain, are pristine samples of Martian rock and soil, collected and returned to Earth for scientific investigation under controlled conditions. (Chapter 4 discusses a Mars sample return mission.) For now, AHL84001 has created a renaissance in the search for life beyond Earth and triggered a wave of new exploration of the Red Planet.

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